Gamma-Ray Detectors For Downhole Applications

ABSTRACT

Methods and related systems are described for gamma-ray detection. A gamma-ray detector is made depending on its properties and how those properties are affected by the data analysis. Desirable properties for a downhole detector include; high temperature operation, reliable/robust packaging, good resolution, high countrate capability, high density, high Z, low radioactive background, low neutron cross-section, high light output, single decay time, efficiency, linearity, size availability, etc. Since no single detector has the optimum of all these properties, a downhole tool design preferably picks the best combination of these in existing detectors, which will optimize the performance of the measurement in the required environment and live with the remaining non-optimum properties. A preferable detector choice is one where the required measurement precision (logging speed) is obtained for all of the required inelastic elements and/or minimization of unwanted background signals that complicate the data analysis.

BACKGROUND

1. Field

This patent specification relates generally to oilfield logging. Moreparticularly, this patent specification relates to methods and systemsfor detecting gamma-rays in downhole applications.

2. Background

Many properties of a subterranean formation may be determined usingdifferent oilfield logging techniques, which may involve one or moretools having a radioisotope source. For example, to locate gas in asubterranean formation, a conventional practice combines data obtainedfrom two tools. One of the tools is a “density” tool, which measures theelectron density of the formation, and the other of the tools is a“neutron porosity” tool, which generally measures the density ofhydrogen in the formation, known as the “hydrogen index” (HI). Based onmeasurements of formation density and hydrogen index, the porosity andpore fluid density of the formation may be determined. For a givenformation fluid density, or gas saturation, a combination of a decreasein the formation density and an increase in the hydrogen index indicatesan increase in the porosity of the formation. Meanwhile, for a givenformation porosity, a combination of a decrease in the formation densityand a decrease in hydrogen index indicates a decrease in the pore fluiddensity and hydrogen content. For pores filled with water and gas or oiland gas, the density and hydrogen index are an indication of the gassaturation (volume fraction of the pores occupied by gas). For poresfilled with gas only, the density and hydrogen index are an indicationof gas density (pressure).

The density and neutron porosity tools for measuring formation densityand hydrogen index may generally employ radioactive sources to obtainformation density and hydrogen index measurements, respectively. Forexample, the density tool may use a source such as ¹³⁷Cs to emitgamma-rays into a formation. Based on a count of gamma-rays scattered bythe formation, the density tool may determine the electron density ofthe formation. Similarly, the neutron porosity tool may use a sourcesuch as ²⁴¹Am-Be to emit neutrons into a formation. A count of neutronsscattered by the formation may yield a hydrogen index measurement. Suchradioisotope sources may be disadvantageous in oilfield tools, as thesources may be heavily regulated by law and their output may diminishover time.

In lieu of such radioisotope sources, an electronic neutron generatormay produce both neutrons and gamma-rays. To do so, the electronicneutron generator may emit neutrons into a formation, which may in turnproduce gamma-rays via inelastic scattering and neutron capture events.A count of gamma-rays produced by inelastic scattering may generallyyield a signal that corresponds to formation density, and a count ofscattered neutrons may generally yield a neutron porosity signal thatcorresponds to the hydrogen index of the formation. However, a count ofgamma-rays produced by neutron capture may also yield a signalcorresponding to the hydrogen index of the formation. Thus, as a countof gamma-rays produced by neutron capture events may overwhelm a countof gamma-rays produced by inelastic scattering, simply counting allscattered gamma-rays may yield a signal that corresponds, at least inpart, to the hydrogen index of the formation. Since such gamma-ray andneutron counts are not independent, the two signals may not enabledetermination of porosity and gas saturation. Other oilfield loggingtechniques may involve spectral analyses of both the inelastic andcapture gamma-rays produced by neutrons emitted into a formation. Asnoted above, it may be possible for detected inelastic gamma-rays to beoverwhelmed by detected neutron capture gamma-rays.

The selection of an optimum detector type for, capture spectroscopy,inelastic measurements, and density measurement has been an issuerequiring many trade-offs. The trade-offs that must be consideredinclude not only the hardware, but how the collected data is processed.

A property that is usually ignored for the inelastic measurement is theaffinity the detector has for interacting with neutrons of variousenergies, including thermal, epithermal, and fast and the production anddetection of associated particles resulting from these interactions. Forsome previous and existing inelastic measurement tools, the number ofdetected events from these neutron reactions can be over 60% of thetotal detected counts. This results in a huge unwanted background signalthat significantly degrades the performance of the desired signal, sincethe unwanted signal must be removed in some way. The degradation inperformance can be due to reduced precision (logging speed) orcomplicating the interpretation of the physics of the measurements suchthat petrophysical usage is limited.

Paper “Response of the Carbon/Oxygen Measurement for an Inelastic GammaRay Spectroscopy Tool, B. A. Roscoe and J. A. Grau, SPE 14460, SPEFormation Evaluation, March (1988) 76-80 and A New Through-TubingOil-Saturation Measurement System”, B. A. Roscoe, C. Stoller, R. A.Adolph, Y. Boutemy, J. C. Cheeseborough, III, J. S. Hall, D. C. McKeon,D. Pittman, B. Seeman, and S. R. Thomas, SPE 21413, presented to the SPEInternational Arctic Technology Conference, Anchorage, Ak., May 29-31,1991; presented to the Middle East Oil Show & Conference, Bahrain, Nov.16-19, 1991 showed the related effects previously discussed on themeasurements (biases) and demonstrated that the bias effects on themeasurement were addressed.

The root cause of the problem has to do with the component materialsused in the radiation detectors where these materials have a highaffinity for interacting with neutrons of various energies, includingthermal, epithermal, and fast and the production and detection ofassociated particles resulting from these interactions. In the early80's, the best downhole detector meeting the needs of the inelasticmeasurement was NaI, which has a very high affinity for neutrons of allenergies (fast, thermal, and epithermal). In the late 80's gadoliniumoxyorthosilicate (GSO or Gd₂SiO₅) became available, which did not havethe problematic fast component other than silicon and oxygen, which werealready present in the formation. However, it had a very large thermaland epithermal neutron component. Bismuth germanate (BGO or Bi₄Ge₃0₁₂)also became available, which had minimal neutron response, but did notoperate at elevated temperatures.

Classically, the thermal neutron component has been removed bysurrounding the detector with a thermal neutron absorbing material, suchas boron, as disclosed in U.S. Pat. No. 4,937,446 for the RST tool ofSchlumberger Technology Corporation. The RST tool design includedsufficient boron to remove all of the thermal neutron signal, and asmuch as reasonably possible, the epithermal neutron signal.

Down-hole density measurements using a sourceless technology have beenproposed. With sourceless technology, the radiation used to produce themeasurement will be produced from an electronic source. The change ofthis source type yields many benefits, but it introduces other issuesthat must be addressed in order to get full benefit from thistechnology.

One of those issues relates to handling the very high instantaneouscount rate that can be present. Actually, the instantaneous detectorcountrate in a tool utilizing an accelerator source can be much higherthan conventional tools for different reasons. First, for aradiochemical source, higher countrates are achieved by using a higheractivity source so transportation issues are more restrictive. Forregulatory concerns still, the maximum activity of these source has tobe limited. An accelerator source can be turned off, so the radiationsafety issues of transportation are minimized. For an acceleratorsource, the activity can be turned up, or down, electronically. Second,some accelerator technologies may need to operate, or the measurementmay want to operate in, a pulsed mode, where all the radiation comes outin a short period of time, e.g. 5 μs. This means that during this burstof radiation, the detector must operate at a very high instantaneouscountrate which many of the standard detectors cannot handle.

Therefore, there is a need for detectors that can handle very highinstantaneous countrates, and still give a signal with the requiredsignal to noise.

SUMMARY

According to some embodiments a system for detecting gamma-rays downholeis provided. The system includes a tool housing adapted and dimensionedto be deployed in a borehole within a subterranean formation; and ascintillator material mounted within the tool housing, and emittinglight when gamma-rays are absorbed, the scintillator material having anassociated decay time of less than about 100 ns and an associatedresonance integral of less than about 100 barns. The system can includea photodetector mounted within the tool housing and adapted so as todetect light emitted by the scintillator material. The system can be ofa type for detecting gamma-rays primarily produced by inelasticscattering, or neutron capture events. The system can also include aelectronic neutron generator source adapted to emit neutrons into thesubterranean formation so as to produce gamma-rays. According to someembodiments, the spacing between the nuclear source and the scintillatormaterial is based in part on the selection of the scintillator material.The system can also include a data processing system adapted andprogrammed to ascertain one or more properties of the subterraneanformation based at least in part on measurements of detected gamma-raysusing an elemental yields type of processing.

According to some embodiments the system is primarily a lanthanum halidematerial such as LaCl₃, LaBr₃ or La(Br,Cl)₃. The tool housing can beadapted to be deployed in the borehole via a wireline cable, or can beadapted to be deployed in the borehole as part of a drill collar.

According to some embodiments a system for detecting gamma-rays that areprimarily produced by neutron capture events downhole is provided. Thesystem includes a tool housing adapted and dimensioned to be deployed ina borehole within a subterranean formation; a scintillator materialmounted within the tool housing, and emitting light when gamma-rays areabsorbed, the scintillator material including an lanthanum halidematerial; and a photodetector mounted within the tool housing andadapted so as to detect light emitted by the scintillator material.

According to some embodiments, a system for detecting gamma-raysdownhole that are primarily produced by inelastic scattering isprovided. The system includes a tool housing adapted and dimensioned tobe deployed in a borehole within a subterranean formation; ascintillator material mounted within the tool housing, and emittinglight when gamma-rays are absorbed; and a photodetector mounted withinthe tool housing and adapted so as to detect light emitted by thescintillator material. The system is adapted to maximize speed ofmeasurement of the system.

According to some embodiments, methods are also provided for detectinggamma-rays downhole.

Further features and advantages will become more readily apparent fromthe following detailed description when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is further described in the detailed descriptionwhich follows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments, in which like referencenumerals represent similar parts throughout the several views of thedrawings, and wherein:

FIG. 1A illustrates a system for determining formation properties usinginelastic or capture gamma-ray measurements, according to someembodiments;

FIG. 1B illustrates a system for determining formation density usinggamma-ray measurements, according to some embodiments;

FIG. 2 illustrates a well-logging operation involving the downhole toolin a surrounding subterranean formation, according to some embodiments;

FIG. 3 is a plot illustrating how the decay time of LaCl can be modifiedby adjusting the concentration of Ce in the matrix, according to someembodiments;

FIG. 4 is a plot illustrating the measured epithermal neutron capturebackground in a limestone formation for six 1″×1″ scintillator crystalsof the types listed in Table 1, according to some embodiments;

FIG. 5 shows a plot 510 that quantifies the tool background measured forthe 6 different crystal materials as a function of porosity, accordingto some embodiments;

FIGS. 6A-C illustrate three different C/O tool configurations based onslightly different performance criterion using a LaCl detector,according to some embodiments;

FIG. 7 is a bar graph illustrating a comparison of C/O logging speedattainable with various configurations of a downhole tool according tosome embodiments, and with existing tools;

FIGS. 8 and 9 are plots comparing measured capture and inelastic spectrafor 5 different detector materials, according to some embodiments

FIG. 10 is a plot showing the absolute capture count rate above 1.6 MeVfor 6 different crystals tested, according to some embodiments;

FIG. 11 is a plot showing the absolute inelastic count rate above 1.6MeV for 6 different crystals tested, according to some embodiments;

FIG. 12 is a plot showing the fraction for capture counts above 1.6 MeV,plotted against crystal density, according to some embodiments;

FIG. 13 is a plot showing the fraction for inelastic counts above 1.6MeV, plotted against crystal density, according to some embodiments;

FIG. 14 is a plot showing the effective 663-keV energy resolutionscalculated for 6 different crystal types compared to actual energyresolutions at 663 keV, according to some embodiments;

FIG. 15 is a plot showing effective 662-keV energy resolutions for 6different crystal types, compared to actual energy resolutions at 6.4MeV, according to some embodiments;

FIG. 16 is a bar chart showing capture spectral quality factors for sixdifferent crystal materials, according to some embodiments;

FIG. 17 is a bar chart showing the inelastic spectral quality factorsrelative to a GSO detector of an RST tool from Schlumberger, accordingto some embodiments;

FIG. 18 is a bar chart showing the estimated capture spectroscopylogging speeds of the tool configurations shown in FIGS. 6A-C;

FIGS. 19A-B illustrate a capture spectroscopy tool utilizing the newlanthanum halide detectors, according to some embodiments; and

FIG. 20 is a bar chart showing the estimated logging speed of the toolconfiguration of FIGS. 19A-B using either LaCl or LaBr for thedetectors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description provides exemplary embodiments only, and isnot intended to limit the scope, applicability, or configuration of thedisclosure. Rather, the following description of the exemplaryembodiments will provide those skilled in the art with an enablingdescription for implementing one or more exemplary embodiments. It beingunderstood that various changes may be made in the function andarrangement of elements without departing from the spirit and scope ofthe invention as set forth in the appended claims.

Specific details are given in the following description to provide athorough understanding of the embodiments. However, it will beunderstood by one of ordinary skill in the art that the embodiments maybe practiced without these specific details. For example, systems,processes, and other elements in the invention may be shown ascomponents in block diagram form in order not to obscure the embodimentsin unnecessary detail. In other instances, well-known processes,structures, and techniques may be shown without unnecessary detail inorder to avoid obscuring the embodiments. Further, like referencenumbers and designations in the various drawings indicated likeelements.

Also, it is noted that individual embodiments may be described as aprocess which is depicted as a flowchart, a flow diagram, a data flowdiagram, a structure diagram, or a block diagram. Although a flowchartmay describe the operations as a sequential process, many of theoperations can be performed in parallel or concurrently. In addition,the order of the operations may be re-arranged. A process may beterminated when its operations are completed, but could have additionalsteps not discussed or included in a figure. Furthermore, not alloperations in any particularly described process may occur in allembodiments. A process may correspond to a method, a function, aprocedure, a subroutine, a subprogram, etc. When a process correspondsto a function, its termination corresponds to a return of the functionto the calling function or the main function.

Furthermore, embodiments of the invention may be implemented, at leastin part, either manually or automatically. Manual or automaticimplementations may be executed, or at least assisted, through the useof machines, hardware, software, firmware, middleware, microcode,hardware description languages, or any combination thereof. Whenimplemented in software, firmware, middleware or microcode, the programcode or code segments to perform the necessary tasks may be stored in amachine readable medium. A processor(s) may perform the necessary tasks.

According to some embodiments, the selection of a gamma-ray detector ismade depending on its properties and how those properties are affectedby the data analysis. Desirable properties for a downhole detectorinclude; high temperature operation, reliable/robust packaging, goodresolution, high countrate capability, high density, high Z, lowradioactive background, low neutron cross-section, high light output,single decay time, efficiency, linearity, size availability, etc. Sinceno single detector has the optimum of all these properties, a downholetool design preferably picks the best combination of these in existingdetectors, which will optimize the performance of the measurement in therequired environment and live with the remaining non-optimum properties.Advantageously, a preferable detector choice is one where the requiredmeasurement precision (logging speed) is obtained for all of therequired inelastic elements and/or minimization of unwanted backgroundsignals that complicate the data analysis.

Some embodiments of the presently disclosed subject matter generallyrelate to systems and methods for well logging using gamma-rays producedby inelastic scattering events (“inelastic gamma-rays”). Counts orspectra of inelastic gamma-rays may indicate a variety of properties ofa surrounding subterranean formation. For example, in combination with ahydrogen index signal, a count of inelastic gamma-rays may enabledetermination of porosity and gas saturation. FIG. 1A illustrates asystem for determining formation properties using inelastic gamma-rays,according to some embodiments. The system 10 includes a downhole tool 12and a data processing system 14. By way of example, the downhole tool 12may be a slickline or wireline tool for logging an existing well, or maybe installed in a borehole assembly for logging while drilling (LWD).The data processing system 14 may be incorporated into the downhole tool12 or may be at the surface at the wellsite or another remote location.

The downhole tool 12 may be constructed so as to improve detection ofgamma-rays produced via inelastic scattering events, while reducingdetection of gamma-rays produced via thermal and epithermal neutroncapture events. Indeed, the downhole tool 12 may provide for a gamma-rayresponse substantially free of epithermal neutron capture background,such that the gamma-ray response is substantially independent of neutronporosity. The particular materials employed in each of the componentsmay be described in greater detail below, with particular reference toFIG. 2. With continued reference to FIG. 1A, the downhole tool 12 may besurrounded by a housing 16 of cobalt-free steel. A neutron source 18 maybe any suitable neutron source capable of emitting neutrons into asurrounding formation to produce inelastic gamma-rays. By way ofexample, the neutron source 18 may be a pulsed electronic neutronsource, such as a Minitron™ by Schlumberger Technology Corporation.Additionally or alternatively, in certain embodiments, the neutronsource 18 may be a radioisotope source capable of emitting fastneutrons. A neutron shield 20 containing elements with high (n,2n) crosssections, such as lead, bismuth or tungsten, may separate the neutronsource 18 from various detectors in the downhole tool 12.

Certain embodiments of the downhole tool 12 may include a neutrondetector 22, which may be surrounded by a cadmium-containing thermalneutron shield 24. The neutron detector 22, which may be a ³He neutrondetector, may primarily detect epithermal neutrons rather than thermalneutrons, since the thermal neutron shield 24 may serve to prevent thepassage of thermal neutrons to the detector 22. The downhole tool 12 mayinclude a neutron monitor, not shown, which is located near the Minitronand detects primarily unscattered neutrons directly from the neutrongenerator. The neutron monitor, which may be a plastic scintillator andphotomultiplier, provides a count rate signal proportional to theneutron output rate from the generator. The downhole tool 12 may includeone or more gamma-ray detectors, illustrated in FIG. 1A as a “near”gamma-ray detector 26 and a “far” gamma-ray detector 28, each of whichmay be surrounded by a housing 30. As should be appreciated, the neargamma-ray detector 26 and the far gamma-ray detector 28 are so named dueto their relative proximity to the neutron source 18. In someembodiments, a scintillator crystal 32 of the near gamma-ray detector 26may be located approximately 8″ to 22″ from the neutron source 18, whilethe scintillator crystal 32 of the far gamma-ray detector 28 may belocated approximately 15″ to 36″ from the neutron source 18.

As noted above, the near gamma-ray detector 26 and the far gamma-raydetector 28 may be contained in respective housings 30. For reasonsdescribed below, each of the housings 30 may contain materialsincorporating ⁶Li, such as lithium carbonate (Li₂C0₃), which maysubstantially shield the gamma-ray detectors 26 and 28 from thermalneutrons without producing thermal neutron capture gamma-rays. Thescintillator crystals 32 of the gamma-ray detectors 26 and 28 may enabledetection counts or spectra of gamma-rays by producing light whengamma-rays are captured by the scintillator crystals 32. For reasonsdescribed below, housings 34 of aluminum alloy or fiberglass maysurround the scintillator crystals 32 to reduce production of epithermalneutron capture gamma-rays. Depending on the application, thescintillator crystals 32 may be chosen to include any of a variety ofmaterials, as described below. Photodetectors 36 may detect lightemitted by the scintillator crystals 32 when a gamma-ray is absorbed,once the light has passed through an optical window 38, to obtain agamma-ray count or spectrum signal.

The signals from the neutron detector 22, the near gamma-ray detector26, and/or the far gamma-ray detector 28 may be transmitted to the dataprocessing system 14 as data 40. The data processing system 14 mayinclude a general-purpose computer, such as a personal computer,configured to run a variety of software, including software implementingall or part of the present technique. Alternatively, the data processingsystem 14 may include, among other things, a mainframe computer, adistributed computing system, or an application-specific computer orworkstation configured to implement all or part of the present techniquebased on specialized software and/or hardware provided as part of thesystem. Further, the data processing system 14 may include either asingle processor or a plurality of processors to facilitateimplementation of the presently disclosed functionality.

In general, the data processing system 14 may include data processingcircuitry 44, which may be a microcontroller or microprocessor, such asa central processing unit (CPU), which may execute various routines andprocessing functions. For example, the data processing circuitry 44 mayexecute various operating system instructions as well as softwareroutines configured to effect certain processes and stored in orprovided by a manufacture including a computer readable-medium, such asa memory device (e.g., a random access memory (RAM) of a personalcomputer) or one or more mass storage devices (e.g., an internal orexternal hard drive, a solid-state storage device, CD-ROM, DVD, or otherstorage device). In addition, the data processing circuitry 44 mayprocess data provided as inputs for various routines or softwareprograms, including the data 40.

Such data associated with the present techniques may be stored in, orprovided by, the memory or mass storage device of the data processingsystem 14. Alternatively, such data may be provided to the dataprocessing circuitry 44 of the data processing system 14 via one or moreinput devices. In one embodiment, data acquisition circuitry 42 mayrepresent one such input device; however, the input devices may alsoinclude manual input devices, such as a keyboard, a mouse, or the like.In addition, the input devices may include a network device, such as awired or wireless Ethernet card, a wireless network adapter, or any ofvarious ports or devices configured to facilitate communication withother devices via any suitable communications network, such as a localarea network or the Internet. Through such a network device, the dataprocessing system 14 may exchange data and communicate with othernetworked electronic systems, whether proximate to or remote from thesystem. The network may include various components that facilitatecommunication, including switches, routers, servers or other computers,network adapters, communications cables, and so forth.

The downhole tool 12 may transmit the data 40 to the data acquisitioncircuitry 42 of the data processing system 14 via, for example, atelemetry system communication downlink or a communication cable. Afterreceiving the data 40, the data acquisition circuitry 42 may transmitthe data 40 to data processing circuitry 44. In accordance with one ormore stored routines, the data processing circuitry 44 may process thedata 40 to ascertain one or more properties of a subterranean formationsurrounding the downhole tool 12. Such processing may involve, forexample, one or more techniques for removing an epithermal neutroncapture background from a gamma-ray count, as described below. The dataprocessing circuitry 44 may thereafter output a report 46 indicating theone or more ascertained properties of the formation, such as porosityand gas saturation, as discussed below. The report 46 may be stored inmemory or may be provided to an operator via one or more output devices,such as an electronic display and/or a printer.

FIG. 2 illustrates a well-logging operation involving the downhole toolin a surrounding subterranean formation, according to some embodiments.In the operation 48 depicted in FIG. 2, the downhole tool 12 has beenlowered into an existing well surrounded by casing 52 in a subterraneanformation 50. The well-logging operation 48 may begin when the neutronsource 18 outputs a burst of neutrons 54 of approximately 14.1 MeV orgreater into the surrounding formation 50. Initially, the burst ofneutrons 54 may collide with nuclei of the formation 50 in inelasticscattering events 56, which causes inelastic gamma-rays 58 to be emittedand the neutrons of the burst of neutrons 54 to lose energy. Followingthe inelastic scattering events 56, the neutrons of the burst ofneutrons 54 may interact with the formation 50 in elastic scatteringevents 60, which causes the neutrons to drop in energy to eV and sub-eVlevels as epithermal neutrons 62 and/or thermal neutrons 64.

The inelastic gamma-rays 58 produced by the inelastic scattering events56 may subsequently Compton-scatter due to electrons in the formation50. Some of the inelastic gamma-rays 58 may ultimately be detected inthe near gamma-ray detector 26 or the far gamma-ray detector 28. Theresulting count of the gamma-rays 58 may be sensitive to formationdensity for at least two reasons. First, the probability that one of theneutrons of the burst of neutrons 54 will interact with a nucleus of theformation 50 may be proportional to the number density of nuclei in theformation 50. Thus, if the formation 50 includes a greater numberdensity of nuclei, a greater number of the neutrons of the burst ofneutrons 54 may inelastically scatter 56 closer to the neutron source18. As such, a lessor number of the inelastic gamma-rays 58 may becreated farther from the near gamma-ray detector 26 or the far gamma-raydetector 28. Second, the inelastic gamma-rays 58 created by theinelastic scattering events 56 may Compton-scatter more frequently ifthe formation 50 is more dense. As a result, the gamma-rays 58 may beless likely to reach the near gamma-ray detector 26 or the far gamma-raydetector 28 before being photoelectrically absorbed by the formation 50.

Either of the above-described effects may lead to a lower gamma-raycount as the density of the formation 50 increases. It should be noted,however, that the former effect is sensitive to the nucleus numberdensity of the formation 50, while the latter effect is sensitive to theelectron number density of the formation 50 or, approximately, the bulkdensity of the formation 50. The nucleus number density may notcorrelate well with bulk density because heavy nuclei and light nucleioften have similar neutron interaction probabilities; however, thecombination of nuclei number density and bulk density obtained from aneutron-gamma measurement may be used in much the same way as aconventional density measurement based on Compton scattering ofgamma-rays.

A complication may arise, however, due to the interaction of theneutrons of the neutron burst 54 with hydrogen in the formation 50.Because elastic scattering 60 occurs primarily due to interactions withhydrogen in the formation 50, the number of low-energy epithermalneutrons 62 and thermal neutrons 64 that reach the vicinity of thegamma-ray detectors 26 and 28 may accordingly be strongly influenced bythe hydrogen index of the formation 50. If one of the epithermalneutrons 62 or thermal neutrons 64 were captured by a nucleus at or inthe vicinity of the near gamma-ray detector 26 or the far gamma-raydetector 28, such as in the casing 52, a neutron capture event 66 mayoccur. Such a neutron capture event 66 may produce a neutron capturegamma-ray 68 that may be detected by the gamma-ray detector 26 or 28.Without correction, neutron capture gamma-rays 68 may completelyoverwhelm the inelastic gamma-rays 58, resulting in gamma-ray countswith the character of neutron porosity measurements, rather than densitymeasurements. Such measurements would be of less use to combine with aneutron porosity measurement to identify gas because the twomeasurements would not be independent.

For this reason, the downhole tool 12 may contain materials carefullychosen to reduce or eliminate neutron capture events 66 occurring in thedownhole tool 12. Moreover, because some neutron capture events 66 mayyet still occur in the downhole tool 12, and because some neutroncapture events 66 may take place externally to the downhole tool 12,various techniques may be employed to reduce or eliminate any remainingneutron capture background from gamma-ray signals of the gamma-raydetectors 26 and/or 28. In particular, the materials chosen and thetechniques employed may reduce or eliminate not only thermal neutroncapture background, but also epithermal neutron capture background, fromgamma-ray signals detected by the gamma-ray detectors 26 and/or 28.Additionally, certain materials in the downhole tool 12, such as thescintillator crystals 32, may be chosen based on additional criteriaparticular to a chosen well-logging application.

Although the discussion above relating to FIG. 1A and FIG. 2 pertainmainly to inelastic measurement modes, according to some embodimentsFIG. 1A and FIG. 2 pertain to capture measurements. For the capturemeasurements, background exists in the form of some epithermalbackground but can be dominated by the thermal neutrons hitting andinteracting with the tool and detector. Classically, this has beenremoved by the use boron shielding, around the detector and preferablyaround the entire tool.

FIG. 1B illustrates a system for determining formation density usinggamma-ray measurements, according to some embodiments. FIG. 1B issimilar to FIG. 1A, except that there is no neutron detector, agamma-ray shield 21 is included instead of a neutron shield, and anx-ray source 19 is included instead of a neutron source. When using anx-ray source 19 for a density measurement instead of the standardradiochemical logging source, the instantaneous countrates experience bythe detectors 26 and 28 can be significantly higher than that observedfrom the radiochemical source. For these applications, selection of adetector with high count rate capabilities (i.e. a fast decay time) willallow the proper processing of these counts to minimize losses anddistortion. A proper selection of the detector type for this application(see Table 12) would suggest a detector with a decay time of less than40 ns would be preferred.

In a preferred embodiment of this invention, a Lanthanum halide(La-halide) detector for the inelastic gamma-ray spectroscopymeasurement is used. The La-halide choice for inelastic spectroscopy isbased on a study evaluating various detectors for the inelasticspectroscopy measurement with regards to optimizing the measurement forprecision (or logging speed) which, at the same time, optimizes theresponse for the most statistically significant number of elementalyields. This evaluation is done by combining several important factorsand their effect on the logging speed (precision), e.g.

(Rel. Logging Speed)˜(Rel.Spectral Quality)*(Rel.Efficiency)*(Rel. MaxCounting Rate)*(Rel. Logging speed due to Bkg.component)

where: (1) Spectral Quality (higher value is better) is a measure of theability to separate, by the least-squares process, different elements ina statistical manner and includes many detector properties, e.g., lightoutput, atomic number, temperature response, peak-to-Compton, size, andresolution; (2) Efficiency (higher value is better) is a measure of thefraction of high-energy gamma-rays absorbed that pass through thedetector and are therefore detected and is related to the detector size,density, and atomic number; (3) Max Counting Rate (higher value isbetter) is a measure of how fast the detector is able to detect andprocess individual gamma-rays that are absorbed in the detector and isbased on the light production and decay properties of the detector; and(4) Relative Improvement due to Neutron Bkg Removal (higher value isbetter) is a measure of the statistical improvement in the measurementdue to a change in the background neutron signal in the detector.

Table 1 illustrates how these properties may vary for scintillatorcrystals of the same size of various types. The last two columns ofTable 1 describe the relative logging speed (higher value is better) forthe detector when all other factors are the same. Specifically, relativelogging speed (no neutron limit) representing when measurements takenwith a scintillator crystal are not neutron limited, meaning that enoughneutrons are produced so as to push the scintillator crystal to itslimit. Relative logging speed (neutron limited) provides a valuerepresenting when measurements taken with a scintillator are neutronlimited, meaning that fewer neutrons are produced than a maximumcapability of the scintillator. The numbers provided in Table 1 arebased on a least-squares processing method, but it should be appreciatedthat the data may also be processed using a standard “windows”processing.

It should be appreciated that, in optimizing a tool containing one ofthe scintillator crystals listed above in Table 1, one would also adjustthe position of the scintillator crystal relative to the neutron source,which may optimize the countrate of the scintillator crystal versus thedegradation in formation response. Therefore, an optimum tool design mayhave an effective relative logging speed somewhere between the values inthe two columns. It should also be noted that the spectral qualityfactor, neutron background term, and efficiency may change as the sizeof the scintillator crystal changes, which may also affect the values inthe last two columns. As apparent in Table 1, spectral quality may notsignificantly impact the values of relative logging speed, but the threeremaining factors may significantly impact the values of relativelogging speed.

TABLE 1 Relative Relative Improvement Logging Relative Relative due toSpeed Logging Relative High- Neutron Relative (no Speed Spectral EnergyBkg Max neutron (neutron Detector Quality Efficiency Removal Countratelimit) limited) LaCl 1.63 0.32 1.26 11.5 16.2 1.23 LaBr 1.62 0.39 0.886.6 6.57 1.07 NaI 1.65 0.32 1.0 1.0 1.0 1.0 GSO 1.84 0.66 0.64 3.9 4.521.48 BGO 2.60 0.61 1.20 0.77 2.25 3.63 LuAP 1.39 0.90 0.57 13.6 12.11.35 LuAG 2.0 0.90 0.57 6.6 10.0 1.94

FIG. 4 is a plot illustrating the measured epithermal neutron capturebackground in a limestone formation for six 1″×1″ scintillator crystalsof the types listed in Table 1, according to some embodiments. Theordinate represents epithermal neutron capture background in units of(ppk), and the abscissa represents porosity in units of porosity units(pu). Curves 410, 412, 414, 416, 418, and 420 plot the measuredepithermal neutron capture background for GSO, LuAP, LaBr, NaI, BGO, andLaCl, respectively. As shown in the plot of FIG. 4, scintillatorcrystals of GSO (gadolinium oxyorthosilicate, or Gd₂SiO₅) and LuAP(LuSiO₃) have the highest epithermal neutron background, followed by thescintillator crystals of LaBr₃ and NaI. The scintillator crystals ofLaCl₃ and BGO have the lowest epithermal neutron capture background. Itshould be noted that the data presented in the plot of FIG. 4 closelyfollow Table 2, which lists the resonance integral of the abovematerials.

Based on the above considerations, the scintillator crystal 32 in thegamma-ray detectors 26 and/or 28 of the downhole tool 12 may includematerials with a resonance integral less than 12, such as BGO (bismuthgerminate, or Bi₄Ge₃0₁₂); and LaCl₃. In particular, the scintillatorcrystals 32 may be formed of a lanthanum-halide scintillator crystal,such as LaCl, for inelastic gamma-ray spectroscopy applications. Withsuch materials, epithermal neutrons 62 that pass through components ofthe downhole tool 12 to reach one of the scintillator crystals 32 may beless likely to cause neutron capture events 66 within the scintillatorcrystals 32. Accordingly, epithermal neutron capture gamma-raybackground may be reduced.

By choosing the scintillator crystal 32 such that the epithermal neutroncapture background is low enough such that the epithermal component doesnot have to be removed by other means, an inelastic gamma-rayspectroscopy measurement may be improved due to various factors. First,if the epithermal capture background is not removed, the statisticalvariance (error) may not be inflated due to the process of subtractingthe epithermal neutron capture background. Second, removal of unwantedcounts may enable the designer of the downhole tool 12 to increaseneutron output from the neutron source 18 or to change the spacing ofthe near and far gamma-ray detectors 26 and/or 28 such that the maximumcount rate capability of each can be utilized for useful counts. Third,some potential complicated neutron physics may be removed from thepetrophysical interpretation. The resulting better performance may beindicated by improved statistical precision and, hence, faster loggingspeed, by the ability to extract more statistically valid inelasticelements, and by the reduction of unwanted physics from the analysis.

To reduce the effect of neutron capture in the gamma-ray detectors 26and/or 28, the scintillator crystals 32 may be surrounded by thermalneutron shields 34. Specifically, to reduce thermal neutron capturegamma-ray production, the thermal neutron shields 34 may employ amaterial enriched with the isotope ⁶Li, which has a relatively highthermal neutron capture cross section, but which also produces primarilycharged particles in lieu of gamma-rays. Candidate materials includelithium carbonate (Li₂C0₃) embedded in epoxy, or metallic lithiumhermetically sealed in a metal can. As illustrated in FIG. 2, when thethermal neutron shields 34 include such a material, thermal neutrons 64that reach the thermal neutron shields 34 may be absorbed without theproduction of neutron capture gamma-rays 68.

Since the thermal component has been classically removed by boronshielding, an embodiment of the invention relates to choosing a class ofdetectors that has a low epithermal component or neutron cross-section.A good indicator of this is the elemental “resonance integral”, which isthe integral cross-section for interaction in the neutron resonanceenergy (“epithermal”) region. The resonance integral for detectormaterials (taken from “Nuclides and Isotopes Chart of the Nuclides”,Knolls Atomic Power Laboratory, 12^(th) edition) of interest is shownbelow:

TABLE 2 Resonance Integral Element s_(a) (barns) Relevant Material Gd49000 400 GSO La 9 12 Lanthanum halide detectors Si 0.16 0.08 GSO, Glassscintillators O 0.003 0.03 GSO, Glass scintillators Na 0.53 32 NaIdetector Cl 33 12 LaCl₃ detector Br 6.8 92 LaBr₃ detector I 6.2 150 NaILu 84 900 Lu-based detectors Y 1.28 1 Y-based detectors (e.g., YAlO₃) Al0.233 0.17 YAP detectors Bi 0.034 0.22 BGO detectors Ge 2.2 6 BGOdetectors Ce 0.6 0.7 Activator in LaCl₃, LaBr₃, GSO

Comparing the data of Table 2 with the neutron performance factor ofTable 1 shows that the resonance integral is a good indicator of whichdetectors will have better or worse performance in the area.

As described above, FIG. 4 shows the measured background frommeasurements in a downhole tool geometry due to epithermal neutrons for6 different detector types. It is plotted as a function of porositysince the porosity will affect how many epithermal neutrons arereflected back at the detector. Here it can be seen that GSO (gadoliniumoxyorthosilicate: Gd₂SiO₅) and lutetium-based scintillators such as LuAP(LuSiO₃), have the highest background contribution followed by LaBr₃ andNaI, and finally with LaCl₃ and BGO having the lowest. Comparing thisinformation with the resonance integrals of the detector elements shownin Table 1, shows the high correlation between the resonance integraland the amount of tool background.

By choosing the appropriate detector where this epithermal contributionis low enough such that the epithermal signal does not have to beremoved by other means, results in better performance of the inelasticmeasurement. This improvement is due to several factors including, 1) noinflation of the statistical variance (error) due to having to subtracta background, 2) removal of unwanted counts allowing the designer toincrease neutron output or change the detector spacing such that themaximum countrate capability of the detector can be utilized for usefulcounts, and 3) removing some potential complicated neutron physics fromthe petrophysical interpretation. The resulting better performance isindicated by 1) improved statistical precision and hence faster loggingspeed, 2) the ability to extract more statistically valid inelasticelements, and 3) reduce unwanted physics from the analysis.

Based on this type of analysis and real tool constraints as far asdetector size, position, current quality of detectors, and detectortypes; the La-halide detectors are clearly superior for the inelasticspectroscopy measurement. This is all put together in FIG. 6 and FIG. 7,which show a comparison of predicted tool performance at elevatedtemperature using different detectors in an inelastic spectroscopy toolconfiguration. These results are shown for both a “least-squares”(labeled “yields”) type of processing, and a “windows” type ofprocessing. Schlumberger performs both types of processing in ourcommercial tools, while it is believed that other companies currentlyonly use the “windows” processing. The RST tool shown in the plot is acommercial service based on a GSO detector. As can be observed, theimprovement for the “yields” processing is more significant with thisdetector selection than it is for the “windows” processing.

FIG. 7 is a bar graph illustrating a comparison of C/O logging speedattainable with various configurations of a downhole tool according tosome embodiments, and with existing tools. An ordinate 758 representsestimated C/O logging speed in units of feet per hour (fph), and anabscissa 760 represents “windows” processing performance, such as bar762, and a least-squares (“yields”) processing performance, such as bar764, for various tools and configurations of the downhole tool 12. TheReservoir Saturation Tool (RST) by Schlumberger using a GSO detector isprovided as a baseline measure of performance. As shown, the estimatedC/O logging speed using the RST for inelastic gamma-ray spectroscopy maybe approximately 160 fph using windows processing and approximately 40fph using least-squares processing.

The performance of various additional configurations of the downholetool 12 are shown in the bar graph of FIG. 7 as tool configurations A,B, and C (corresponding to the configurations of tool 610 in FIG. 6A,tool 630 in FIG. 6B, and tool 650 in FIG. 6C, respectively). FIGS. 6A-Cillustrate three different C/O tool configurations based on slightlydifferent performance criterion using a LaCl detector, according to someembodiments. In FIG. 6A, the tool 610 in configuration A represents a 111/16″ configuration of the downhole tool 12 (shown in FIGS. 1A and 2)in which the near gamma-ray detector 612 (corresponding to detector 26in FIGS. 1A and 2) includes a scintillator crystal 614 (corresponding tocrystal 32 in FIGS. 1A and 2) of LaCl of approximately 1″×3″, locatedapproximately 8.5″ from the neutron source 620 (corresponding to source18 in FIGS. 1A and 2), and the far gamma-ray detector 616 (correspondingto detector 28 in FIGS. 1A and 2) includes a scintillator crystal 618(corresponding to crystal 32 in FIGS. 1A and 2) of LaCl of approximately1″×4″, located approximately 18.6″ from the neutron source 620. Theneutron source 620 may be configured to output neutron bursts with anominal output strength for a standard device. The estimated C/O loggingspeed using the configuration of tool 610 for inelastic gamma-rayspectroscopy may be approximately 210 fph using windows processing andapproximately 240 fph using least-squares processing.

In FIG. 6B, the tool 630 in configuration B represents a 1 11/16″configuration of the downhole tool 12 shown in FIGS. 1A and 2, in whichthe near gamma-ray detector 632 (corresponding to detector 26 in FIGS.1A and 2) includes a scintillator crystal 634 (corresponding to crystal32 in FIGS. 1A and 2) of LaCl of approximately 1″×4″, locatedapproximately 11″ from the neutron source 640 (corresponding to source18 in FIGS. 1A and 2), and the far gamma-ray detector 636 (correspondingto detector 28 in FIGS. 1A and 2) includes a scintillator crystal 638(corresponding to crystal 32 in FIGS. 1A and 2) of NaI of approximately1″×6″, located approximately 24″ from the neutron source 640. Theneutron source 640 may be configured to output neutron bursts 54 of anominal amount for a standard device. The estimated C/O logging speedusing the configuration B of tool 630 for inelastic gamma-rayspectroscopy may be approximately 180 fph using windows processing andapproximately 205 fph using least-squares processing.

In FIG. 6C, the tool 650 in configuration C represents a 2½″configuration of the downhole tool 12 shown in FIGS. 1A and 2, in whichthe near gamma-ray detector 652 (corresponding to detector 26 in FIGS.1A and 2) includes a scintillator crystal 654 (corresponding to crystal32 in FIGS. 1A and 2) of LaCl of approximately 1.2″×1″, locatedapproximately 8.5″ from the neutron source 660 (corresponding to source18 in FIGS. 1A and 2), and the far gamma-ray detector 656 (correspondingto detector 28 in FIGS. 1A and 2) includes a scintillator crystal 658(corresponding to crystal 32 in FIGS. 1A and 2) of LaCl of approximately1.6″×3″, located approximately 17.5″ from the neutron source 660. Theneutron source 660 may be configured to output neutron bursts 54 ofapproximately twice the nominal neutron output of a smaller diametertool, approximately double the output of the neutron source 18 of thetools A and B. The estimated C/O logging speed using tool 650 inconfiguration C for inelastic gamma-ray spectroscopy may beapproximately 350 fph using windows processing and approximately 420 fphusing least-squares processing.

Further detail on spectral analysis and elemental standards derivationswill now be provided. There are quite a few tool-performance parametersthat can be estimated by making measurements with a minitron-basedmockup tool. Among the more important are described the followingparagarphs.

The Spectral Quality for Both Capture and Inelastic Spectra.

This is a measure of the contribution to the statistical uncertainty dueto how difficult it is to separate one element from another in a complexspectrum. This contribution can be estimated by deriving a complete setof elemental standard spectra and then extract the diagonal element ofthe weighted-least-squares error matrix.

The Absolute Efficiency for High-Energy Gamma Rays (>1.6 MeV).

This parameter is useful especially for minitron-limited tool designs.Typically, standard spectral analysis only includes events above 1.6 MeVso a higher density crystal will detect more such events per neutronemitted.

The Fraction of Counts Above 1.6 MeV.

This parameter is useful since the counts above 1.6 MeV are the measuredspectroscopy signal. The counts below 1.6 MeV are most of the counts andthey contribute to the total count rate, which is thethroughput-limiting factor.

The Energy Resolution for High-Energy Gamma Rays.

This parameter is very useful for spectral quality since the capture andinelastic spectra are analyzed all the way out to 8-9 MeV; whereas, itis most common to measure energy resolution at low energies (e.g. 662keV) with radioactive chemical sources.

Fast-Neutron Crystal Background.

This can be a large contribution to the inelastic measurement for somecrystals, and it may be difficult or impossible to calculate accuratelysince the fast-neutron gamma-production cross-sections are not well knowfor many of the elements found in the crystals we are studying.

The measurements made with a mockup tool for most of the crystals arelisted below in Table 3. Most of these measurements were made for thepurpose of deriving the capture and inelastic elemental standardspectra, with the primary element extracted from each measurement listedin the table in the columns labeled capture and inelastic. The oil-sandmeasurement was a recent addition for the purpose of estimatingoil-saturation sensitivity and was not included in every measurementset. All told, 15 sets of measurements have been made as of this writingwith 10 different crystals.

TABLE 3 List of measurements with the mockup tool for crystalevaluations Lithology BHD Por Salinity Capture Inelastic Time ExtraStuff/Comments Inf Water x 100 Fresh H O 3 hr Inf Water x 100 200 Cl 3hr ppk Inf Water x 100 Fresh CrNi 3 hr Stainless borehole liner in watertank Inf Water x 100 Fresh Fe Fe 3 hr Iron mesh around tool Inf Water x100 Fresh Mg Mg 3 hr Magnesium around tool Inf Water x 100 Fresh Al 3 hrAluminum around tool Sand 8 14.2 Fresh Si Si 3 hr Lime 8 13.2 Fresh CaCa 3 hr Lime 8 29.3 Fresh Tool 3 hr Lime 8 0 Fresh Tool 3 hr Dolo 8 11.7Fresh Mg Mg 3 hr Inf Oil x 100 Fresh C 3 hr Sand 8 16 Oil 3 hr For oilsensitivity

FIGS. 8 and 9 are plots comparing measured capture and inelastic spectrafor 5 different detector materials, according to some embodiments. Thespectra are all on an absolute scale, normalized to a nominal minitronoutput. Activation background has been subtracted from the capturespectra and capture background from the inelastic spectra. Differencesin energy resolution and absolute efficiency for the different materialsare readily apparent from the plots 810 in FIGS. 8 and 910 in FIG. 9.

Total Counting Efficiency Above 1.6 MeV.

Due to the vastly different densities of the crystals tested there is asignificant difference in the counting efficiencies for the high-energygamma rays detected in neutron-induced gamma-ray spectroscopymeasurements. This is useful for minitron-limited tool designs, or fordesigns where it is desirable to run the minitron at lower outputs toachieve longer lifetimes. FIG. 10 is a plot showing the absolute capturecount rate above 1.6 MeV for 6 different crystals tested, according tosome embodiments. Plot 1010 shows the absolute capture count rate above1.6 MeV plotted as a function of crystal density. The response isreasonably linear, with LuAP showing a significant efficiency advantage,and LaBr being measurably better than LaCl. FIG. 11 is a plot showingthe absolute inelastic count rate above 1.6 MeV for 6 different crystalstested, according to some embodiments. As can be seen from plot 1110,the trend with density here is not as linear, most likely due to theperturbing effect of different sensitivities to epithermal neutrons inthe different detector materials, which would inflate the count ratesfor GSO and LuAP.

Fraction of Counts Above 1.6 MeV.

The fraction of counts above 1.6 MeV is an useful parameter forthroughput-limited tool designs; i.e., throughput is limited by thetotal count rate, but only events above 1.6 MeV are spectrally analyzed.So the analyzed count rate would be the total throughput count rate(which is ultimately limited by the decay time of the light output)times the fraction above 1.6 MeV. FIG. 12 is a plot showing the fractionfor capture counts above 1.6 MeV, plotted against crystal density,according to some embodiments. As can be seen from plot 1210, there is areasonably linear trend. FIG. 13 is a plot showing the fraction forinelastic counts above 1.6 MeV, plotted against crystal density,according to some embodiments. From plot 1310, the trend with density isnot quite as clear as the capture case, probably again due to theperturbing effect of the differing fast-neutron crystal backgroundcontributions.

Effective Energy Resolution.

The energy resolution measured at 662 keV is generally not a very goodpredictor of spectral quality for neutron-induced gamma-ray spectroscopymeasurements because the energy range of interest is much larger,typically between 1.6 MeV and about 9 MeV. If the peak widths of allcrystals increased linearly with the square root of the peak energy, aspredicted by simple counting statistics based on the number of electronsproduced, then it wouldn't matter because everything would scaleuniformly. For some crystals, however, the peak widths increase muchmore quickly at higher energies. Since we are all familiar withevaluating crystals based on their resolution at 662 keV, it isconvenient to define an effective 662-keV resolution for each crystalsuch that its spectral quality would be equivalent to that of a crystalof that 662-keV resolution which obeys the ideal square-root resolutiondependence. FIG. 14 is a plot showing the effective 662-keV energyresolutions calculated for 6 different crystal types compard to actualenergy resolutions at 662 keV, according to some embodiments. In plot1410, six 1″×1″ crystals evaluated here versus their measuredresolutions at 662 keV. LaCl, LaBr, NaI, and BGO plot very close to the1:1 line, indicating that they follow the ideal square-root dependencefairly closely. By contrast, the effective resolutions of GSO and mostnotably LuAP, are quite a bit higher than their actual 662-keVresolutions. The tendency for the energy resolution of LuAP crystals todeteriorate at high energy is a drawback to using this material forneutron-induced capture spectroscopy measurements. A better predictor ofspectral quality would be the energy resolution of a peak toward themiddle of the 1.6-9 MeV analysis region. One such convenient peak wouldbe the 6.4-MeV line from calcium. FIG. 15 is a plot showing effective662-keV energy resolutions for 6 different crystal types, compared toactual energy resolutions at 6.4 MeV, according to some embodiments. Itcan be seen from plot 1510 that there is a fairly linear dependence ofthe effective resolution versus the resolution at 6.4 MeV. Using 3 timesthe resolution of the 6.4-MeV calcium peak would be a convenientapproximation of the effective energy resolution.

Capture Standards Derivations.

Table 4 lists the measurements used to derive the capture elementalstandards, along with the elemental yields, expressed in parts perthousand of the total spectrum, that were extracted from thesemeasurements for the 1×1″ LaCl series. Yields highlighted in underlineindicate the primary measurement used to derive that particularelemental standard. As will be described below, the standard spectra forsulfur, gadolinium, potassium, and titanium were generated with the aidof a general-purpose Monte Carlo N-Particle (MCNP) code that can be usedfor neutron, photon, electron, or coupled neutron/photon/electrontransport developed by Los Alamos National Laboratory. The standardlabelled hfmb, hydrogen formation minus background, is also acalculation and represents the difference in spectral shape of thehydrogen standard depending on whether the gamma ray originated for theformation or the borehole.

TABLE 4 Capture elemental yields for the LaCl measurements used forstandards derivation. Lithology Por Salinity chy cfe crni csi cca cmgcchl calu csul cgd ck cti hfmb Water 100 Fresh 984    0  0  0    0  −1 16    1 0 1 −1 0 0 Water/Fe 100 Fresh 605   386  0  0    0    0  10  −10 1 0 0 0 Water/SST 100 Fresh 892    53 40  0    0    0  15    0 0 1 −10 0 Sand 14.2 Fresh 357    62 47 528    0    0  6    0 0 0 0 0 0 Lime 0Fresh 433    5  6  19   536    1  31    10 −2 −13 −2 −4 −20 Lime 13.2Fresh 357    2  2  0   633    0  6    0 0 0 0 0 0 Lime 29.3 Fresh 426   1  1  3   556    0  11  −2 −1 −1 0 −1 8 Dolo 11.7 Fresh 367    73 42 27   394   55  25    3 −6 20 12 1 −12 Water/Mg 100 Fresh 945  −1  1  6 −1   19  9    9 −3 8 4 2 1 Salt 100 200 ppk 100    0  0  0    0    0899    0 0 0 0 0 0 Water/Al 100 Fresh 885    0  0  0    0    0  14   1000 1 0 0 0

Standards derivation uses a spectral stripping process to solve 2 or 3equations at a time for 2 or 3 unknowns, with the unknowns being theelemental contributions to each measurement. The measurements are chosenso that they each contain a minimum number of different elements so thatthe problem is reasonably tractable. In addition to determining thecorrect stripping factor, the individual measurements also need to begain matched and occasionally resolution matched. The PMT gain for thesemeasurements was controlled using a Cs peak in the burst-off-backgroundspectrum, however additional adjustments of up to a few tenths of apercent are still necessary for optimal gain matching. Since everymeasurement contains a distinct 2.2 MeV hydrogen peak we have gainmatched all of the measurements to the gain of the water tankmeasurement. Occasionally some additional source of noise would bepicked up, requiring a slight amount of resolution matching, usually inthe form of a small deconvolution of the more noisy measurement.

Since there is a certain amount of subjectivity involved in thestandards generation process, one concern may be how much consistencycould be achieved, especially while deriving 15 sets of capture andinelastic standards over an extended period. After several less thansatisfying attempts, the minitron monitor information was used instead.First, all measurements were normalized by time and the minitronmonitor. Since the only thing changing between sets of measurements isthe crystal, all of the stripping factors should actually be identical,or very nearly so. A stripping template was developed, first using thereally good-resolution LaCl series. The template included all of thestripping operations needed together with all of the stripping factors.This template was used as a start for each subsequent crystal series.Changes were then made to the stripping factors only when they wereclearly incorrect. The changes needed were only a few percent here andthere, so confidence is high that consistency has been achieved. Notethat it is not necessary that the monitor calibration be consistentacross different sets of measurements, but rather only that it beconsistent within each set.

Inelastic Standards Derivations.

Table 5 lists the measurements used to derive the inelastic elementalstandards, along with the elemental yields, expressed in parts perthousand of the total spectrum, which were extracted from thesemeasurements for the 1×1″ GSO series. Yields highlighted in underlineindicate the primary measurement used to derive that particularelemental standard. The standard spectrum for sulfur was calculatedusing the same techniques describe above for the capture standards.

TABLE 5 Inelastic elemental yields for the GSO measurements used forstandards derivation. Lithology Por Salinity ic io isi ica ife itb imgisul cor wcor Oil 100 Oil 593  36  0  0 171 201  0 0 16.690 6.680 Water100 Fresh  0 580  0  0 184 236  0 0  0.000 1.235 Water/Fe 100 Fresh  0478  0  0 290 232  0 0  0.000 1.314 Sand 14.2 Fresh  0 292 248  0 147313  0 0  0.000 1.480 Lime 0 Fresh  40 250  0 134 131 445  0 −1  0.1621.970 Lime 13.2 Fresh  47 314  0 161 149 328  0 0  0.150 1.802 Lime 29.3Fresh  46 351  0 154 157 292  0 −1  0.132 1.708 Dolo 11.7 Fresh  56 317 6  87 194 280 65 −5  0.175 1.739 Water/Mg 100 Fresh  0 506  0  0 178232 85 0  0.000 1.297

Inelastic Tool Background Differences.

Inelastic tool background, which is due to fast neutrons interactingdirectly with the crystal itself, can be a significant contribution tothe burst spectrum. The reactions of importance are generally eitherinelastic scattering or epi-thermal capture. Inelastic tool backgroundwas derived consistently for the different crystal types using the 0-puand 29-pu limestone measurements and the 100-pu water-tank measurement.The same stripping equation was used for all the standard sets:

Tool Background=0-PU Lime−1.18*29-PU Lime+0.18*100-PU Water

The motivation for this derivation is that fast-neutron tool backgroundis maximized at the lowest porosities, so if the calcium, carbon, andoxygen contributions can accurately be eliminate from the 0-pu limestonemeasurement, the tool contribution will remain. The first subtraction of1.18 times the 29-pu lime removes the calcium and carbon contributions,but it over subtracts oxygen, which is then added back using the watertank measurement. This result actually includes the crystal contributionplus a smaller contribution from the pressure housing, which issubtracted in a later step using the iron signal derived from theWater/Fe measurement.

FIG. 5 shows a plot 510 that quantifies the tool background measured forthe 6 different crystal materials as a function of porosity, accordingto some embodiments. Note that for LuAP and GSO, the crystal backgroundis nearly 50% of the signal at low porosity. Most of the crystalbackground for these two materials is due to epi-thermal captures, whichcan be reduced somewhat by wrapping enriched boron around the crystal asis done for RST. FIG. 4 shows the result of wrapping enriched boronaround LuAP, GSO, LaBr, and LaCl. A significant reduction in crystalbackground is achieved for GSO, LuAP, and LaBr confirming the largeepi-thermal contributions for these materials. The extra boron hadabsolutely no effect on the already low LaCl background, which impliesthat there is no epi-thermal component to its background.

Spectral Contribution to the Capture Statistical Uncertainties.

Having generated complete sets of elemental standard spectra the elusivespectral contribution to the statistical uncertainties can be extractedfrom the diagonal element of the weighted-least-squares error matrix.This calculation implicitly accounts for the similarities anddifferences in the shapes of the elemental standard spectra that definethe precision with which a complex spectrum can be separated intoelemental contributions.

As an example, Table 6 is a representation of the WLS error matrix forthe 1×1″ LaCl capture analysis. The diagonal element is the spectralcontribution to the statistical uncertainty of each elemental yield,with the total yield uncertainty then being equal to this diagonalelement divided by the square root of the number of counts in theanalysis window (assuming no background subtraction). The off-diagonalelements represent the statistical correlation between element pairs inpercent.

TABLE 6 WLS error matrix for the capture analysis of LaCl. chy csi ccacchl cfe crni cmg calu csul cgd ck cti chy    1.565   40 −17  −9  −3 −12  27  −4  −8 −64   19   17 csi   40    3.766  −8   11   22  −8 −24 −19−12    0 −37 −15 cca −17  −8    4.146 −25    3    5  −5   21  −6 −11 −16−56 cchl  −9   11 −25    5.171 −32 −26  −6 −13    8    4 −28 −46 cfe  −3  22    3 −32    2.837 −19 −15 −70  −5   20   11    2 crni −12  −8    5−26 −19    1.018 −17    6 −19   13   17   13 cmg   27 −24  −5  −6 −15−17    4.700  −6    9 −71   19   13 calu  −4 −19   21 −13 −70    6  −6   4.350   11    4 −32    4 csul  −8 −12  −6    8  −5 −19    9   11   2.679    6 −48    4 cgd −64    0 −11    4   20   13 −71    4    6   6.486 −43    7 ck   19 −37 −16 −28   11   17   19 −32 −48 −43   6.852   21 cti   17 −15 −56 −46    2   13   13    4    4    7   21   2.027

Table 7 shows just the diagonal elements of the WLS error matrix for thesix crystals measured for this study. The calculations for LuAP wereperformed with and without the high-energy tail elimination—clearly thetail has a significant effect. For comparison, the same information forthe 3×4″ BGO detector of the ECS are included in the bottom row of thetable.

TABLE 7 Spectral contribution to the statistical uncertainties forcapture elements. Xtal chy csi cca cchl cfe crni cmg calu csul cgd ckcti LaCl 1.565 3.766 4.146 5.171 2.837 1.018 4.700 4.350 2.679 6.4866.852 2.027 LaBr 1.424 3.234 3.605 4.397 2.494 0.876 4.146 3.683 2.5806.026 6.349 1.904 NaI 1.494 3.517 3.768 4.917 3.126 1.001 4.754 3.4263.020 6.372 6.529 1.830 GSO 1.564 5.966 5.327 8.923 7.172 1.397 6.5619.111 5.020 8.335 10.548 3.322 BGO 1.208 3.278 3.148 4.849 3.760 1.0194.634 5.250 3.419 6.185 7.586 2.087 LuAP 1.906 10.174 8.711 16.49410.174 1.746 10.665 12.935 8.585 12.042 14.818 5.411 No. 1.382 6.2476.214 10.972 7.117 1.418 6.470 9.398 5.601 8.167 11.317 3.656 Tail ECS1.025 1.962 2.255 3.816 2.440 2.496 3.557 3.001 2.130 6.747 5.008 1.448

In order to distill the information in Table 7 into a single number foreach crystal a parameter called spectral quality is defined, whichscales as logging speed and is calculated from the most importantelements Si, Ca, and Fe. FIG. 16 is a bar chart showing capture spectralquality factors for six different crystal materials, according to someembodiments. In chart 1610, the spectral qualify factor has beencalculated relative to the 3×4″ BGO detector of the ECS, for the six1″×1″ crystals. GSO and LuAP suffer notably in this comparison due totheir poor resolution at high energies.

Spectral Contribution to the Inelastic Statistical Uncertainties.

Table 8 is a representation of the weighted-least-squares error matrixfor the LaCl inelastic analysis. As before, the diagonal elements arethe spectral contribution to the statistical uncertainty and the offdiagonal elements represent the statistical correlation between elementpairs in percent.

TABLE 8 WLS error matrix for the inelastic analysis of LaCl. ic io isiica ife itb img isul ic    2.170   11 −11 −60   20    0    3   36 io  11    2.901 −12 −19 −21 −12    5   56 isi −11 −12    5.577   42 −61−20 −41 −19 ica −60 −19   42    7.406 −69 −17 −19 −24 ife   20 −21 −61−69    8.935    4  −7  −4 itb    0 −12 −20 −17    4    3.293  −2 −40 img   3    5 −41 −19  −7  −2    3.997    6 isul   36   56 −19 −24  −4 −40   6    1.939

Table 9 includes just the diagonal elements of the WLS error matrix foreach of the 6 crystals studied here. For this analysis the lower limitwas set at 1.1 MeV to pick up the prominent gamma ray from magnesium,and magnesium and sulfur were included in the analysis. For Table 10,the lower analysis limit was set to the normal 1.6 MeV and magnesium andsulfur were not included. This is the normal analysis mode for C/Ologging.

TABLE 9 Spectral contribution to the statistical uncertainties forinelastic elements. Analysis limit lowered to 1.1 MeV for Mgsensitivity. Xtal ic io isi ica ife itb img isul LaCl 2.170 2.901 5.5777.406 8.935 3.293 3.997 1.939 LaBr 2.217 4.146 3.685 5.997 7.940 7.7143.985 1.751 NaI 2.226 3.049 5.931 8.948 9.605 8.396 5.202 2.113 GSO2.264 2.529 4.182 7.349 7.687 6.167 3.762 2.124 BGO 1.758 2.586 4.0735.867 6.399 5.405 3.165 1.787 LuAP 2.664 2.942 4.180 8.471 8.939 6.3304.256 2.246 No Tail 2.287 2.652 3.588 7.186 7.373 5.509 3.182 1.936

TABLE 10 Spectral contribution to the statistical uncertainties forinelastic elements. Lower analysis limit set at 1.6 MeV Xtal ic io isiica ife itb LaCl 1.997 2.647 4.665 6.461 8.707 2.496 LaBr 2.025 3.9163.089 5.459 7.432 7.188 NaI 1.919 2.653 4.356 7.526 8.293 5.849 GSO2.189 2.273 3.701 5.427 8.828 8.239 BGO 1.598 2.248 3.229 5.104 7.6505.988 LuAP 2.352 2.522 3.824 6.958 9.576 6.539 No Tail 2.078 2.341 3.1126.057 8.175 5.653

FIG. 17 is a bar chart showing the inelastic spectral quality factorsrelative to a GSO detector of an RST tool from Schlumberger, accordingto some embodiments. More specifically, chart 1710 shows the inelasticspectral quality factors for the 1″×1″ crystals relative to the 1″×4″GSO detector of Schlumberger's RST tool. In this case only the carbonyield is included in the quality calculation because its uncertaintydominates the bottom-line oil saturation uncertainty. It is interestingthat the spectral quality for GSO and LuAP suffered greatly in thecapture analysis due to the poor energy resolution of these crystal athigh energy, however there is much less of a resolution effect for theinelastic analysis. In this case the poorer energy resolutions of GSOand LuAP are compensated by the better full-energy efficiencies of thesematerials relative to LaCl, LaBr, and Nat BGO is the clear winner herehaving both reasonably good high-energy resolution and good full-energyefficiency. However, note that this is just the spectral contribution tothe statistical uncertainty—the significant drawbacks to BGO are that itcan't count nearly as fast as the other crystals and it should be keptcool while logging.

Tool Applications.

Based on the measurements performed as described above, according tosome embodiments, it is possible to make performance estimates ondifferent potential tool configurations. For the capture measurements,these parameters are far less important. The estimates include majorfactors that will affect a tool measurement, including, total detectorefficiency, detector high energy efficiency, detector resolution at roomtemperature and high temperature, variance inflation factors due tospectral shapes, dynamic ranges, and more. The detector performance isbased on the performance observed in the one or two samples of detectorthat were available for the analysis. Therefore, some of theextrapolations/interpolations are large. As a result, it has been foundthat a laboratory mock-up is useful in the tool development so that thetool configuration can be properly optimized and the magnitude of theextrapolations/interpolations reduced.

FIGS. 6A-C illustrate three different C/O tool configurations based onslightly different performance criterion using a LaCl detector,according to some embodiments. Tools 610 in FIG. 6A and tool 650 in FIG.6C are both optimized to maximize C/O logging speed and to have aboutthe same borehole sensitivity as the Schlumberger RST-A tool. Theconfiguration of tool 610 in FIG. 6A is for a 1 11/16 inch tool whilethe configuration of tool 650 in FIG. 6C is for a 2½ inch tool (andassumes twice the neutron output of the smaller tool 610). C/Omeasurement statistics (and hence logging speed) come from both of thedetectors used in tools 610 and 650; however, the far detector on tool610 does not add that much to the overall logging speed as shown in FIG.7 for the various configurations. Because of this, a better 1 11/16 inchtool design might be configuration B where all of the C/O statisticscome from the near detector, and the far detector is moved farther awayto improve its gas sensitivity. All of these configurations will alsoprovide Spectroscopy and Sigma answer products. Of course, there aremany more tool variations possible depending on the tool requirementselection.

FIG. 7 shows the estimated logging speeds for both an “elemental yields”and a “windows” type interpretation. As mentioned earlier, since most ofthe C/O signal from tool configuration A (tool 610 of FIG. 6A) comesfrom the near detector, removing the far detector from the C/Omeasurement of tool configuration B (tool 630 of FIG. 6B) does notdegrade the C/O logging speed very much. For the large tool,configuration C (tool 650 of FIG. 6C), both the near and far detectorsadd to the C/O logging speed; hence, tool configuration C has a muchfaster logging speed than the smaller tools.

Comparing the performance of a “windows type” and an “elemental yields”type of analysis shows that the “elemental yields” type of processingbenefits more from the new detectors than the “windows” type ofprocessing.

For Schlumberger's RST C/O tool, Schlumberger provides an “alpha factor”processed result that combines the precision of a windows measurementwith the accuracy of the elemental yields type measurement. For water inthe borehole, this provides significant perceived increased loggingspeed; however, for oil in the borehole, the advantages of alphaprocessing are less. The algorithm to combine windows and elementalyields processing is simple in concept, but quite involved when onetries to figure out how the statistics propagate through the analysis.This combination would benefit from more data for estimates than iscurrently available, and estimates were not made. However; it has beenfound that for the LaCl detector tool configurations shown, that theelemental yields processing provides better logging speeds than thewindows processing. Thus, alpha processing will not improve these toolconcepts. However, there might be some advantages in other toolconfigurations with other detector materials using the techniques andevaluation methodologies as described herein.

FIG. 18 is a bar chart showing the estimated capture spectroscopylogging speeds of the tool configurations shown in FIGS. 6A-C. Forreference, the logging speeds of the RST tool and ECS tool capturemeasurements are shown in plot 1810. As is evident, the use of LaCl inthe new tool configurations provides significant improvements in loggingspeed over the RST tool's GSO detectors. The data shows that if onelogged with these tools for C/O data, the tools will provide capturespectroscopy results of better quality than the current ECS tool loggingat 1800 fph. This means that an excellent integratedlithology/saturation log would be possible with the new tools.

FIGS. 19A-B illustrate a capture spectroscopy tool utilizing the newlanthanum halide detectors, according to some embodiments. The tool 1910uses a large neutron generator with 1920 closely spaced detectors thatare 1-inch in diameter. The detector 1912 uses three smaller diametercrystals 1914, 1916 and 1918. It has been found that using threeseparate small diameter crystals provides better performance than asingle large diameter detector. FIG. 20 is a bar chart showing theestimated logging speed of the tool configuration of FIGS. 19A-B usingeither LaCl or LaBr for the detectors. The ECS is also included for areference point. As the chart 2010 indicates, a significantly improvedcapture spectroscopy tool could be built with these new detectormaterials with LaBr providing the best performance. Even though this isshown as improvements in logging speed in chart 2010, the improvementscan be translated to improved precision at the lower ECS logging speed.This improves the statistical precision of additional elements, such asAl and Mg. This could have significant impact in some applications, suchas in carbonates. Furthermore, major advantages in tool design andoperating logistics could be realized due to the excellent hightemperature performance of the new halide detectors compared to thecurrent ECS. The present ECS has a Dewar flask and requires pre-coolingof the BGO detector before logging. Neither a Dewar nor pre-coolingwould be required for the new halide detectors.

Further detail of embodiments generally relating to the use of La-halidedetectors for the capture gamma-ray spectroscopy measurement will now beprovided. Experiments have been carried out evaluating various detectorsfor the capture spectroscopy measurement with regard to optimizing themeasurement for precision (or logging speed) which, at the same time,optimizes the response for the most statistically significant number ofelemental yields. A number of factors and their effect on the loggingspeed (precision) have been analyzed.

(Rel. Logging Speed)−(Rel.Spectral Quality)*(Rel.Efficiency)*(Rel. MaxCounting Rate)

Where: Spectral Quality (preferably a higher value) is a measure of theability to separate, by the least-squares process, different elements ina statistical manner and includes many detector properties, e.g., lightoutput, atomic number, temperature response, peak-to-Compton, size, andresolution; Efficiency (preferably a higher value) is a measure of thefraction of high-energy gamma-rays absorbed that pass through thedetector and are therefore detected and is related to the detector size,density, and atomic number; and Max Counting Rate (preferably a highervalue) is a measure of how fast the detector is able to detect andprocess individual gamma-rays that are absorbed in the detector and isbased on the light production and decay properties of the detector.

Table 11 shows how these properties vary for several detectors (havingthe same size) of various types. The last two columns of Table 11 givethe relative logging speed (preferably a higher value) for the detector(assuming everything else is the same) and also assuming that themeasurement is either neutron limited or not, i.e., if you can produceenough neutrons to push the detector to the limit. In optimizing a tool,the detector position would also be adjusted relative to the sourceoptimizing the count-rate versus the degradation in formation response.Therefore, the optimum tool design would have a relative logging speedsomewhere between the values in the two columns. Note, the spectralquality factor and efficiency will change as detector size changes, thusaffecting the values in the last two columns.

TABLE 11 Relative Relative Logging Relative Relative Relative LoggingSpeed Detec- Spectral High-Energy Max Speed (no (neutron tor QualityEfficiency Countrate neutron limit) limited) LaCl 0.38 0.39 11.5 10.70.93 LaBr 0.515 0.48 6.6 9.7 1.5 NaI 0.400 0.40 1.0 1.0 1.0 GSO 0.1280.72 3.9 1.7 0.6 BGO 0.303 0.78 0.77 0.8 1.5 LuAP 0.112 0.96 13.6 5.70.7 LuAG 0.49 0.75 6.6 10.9 2.3

Based on this type of analysis and real tool constraints as far asdetector size, position, current quality of detectors, and detectortypes; the La-halide detectors have been found to be superior for thecapture spectroscopy measurement in many applications. Chart 2010 inFIG. 20 shows a comparison of predicted tool performance at elevated androom temperature using different detectors in a capture spectroscopytool configuration. The current state of the art tool, ECS, uses a BGOdetector that will not work at elevated temperature (and therefore mustbe put in a Dewar), its performance is only shown for room temperature.As can be seen, the performance using La-halide detectors issignificantly improved over that of the ECS with about a factor of twoimprovement in logging speed for LaCl3 and over a factor of threeimprovement for LaBr3. This is true even if the detector is used at hightemperature. A further advantage to the above embodiment is that thedetector does not have to be put in a Dewar.

Further details of embodiments relating generally to downhole densitymeasurements will now be provided. Currently, NaI is the detector ofchoice for downhole density applications because of its good hightemperature capabilities, low cost, and reasonable performance.According to some embodiments, a method to identify detectors that willallow the faster counting of gamma-ray pulses for downhole applicationsis provided. For this application, one important detector characteristicis the detector decay time, which is shown in Table 12 for severaldifferent detectors.

To first order, the countrate capability of the detector is directlyproportional to the decay time, the faster the decay time, the better.It has been found that a decay time of less than about 100 ns issuitable for some applications, according to some embodiments. Forexample, LaCl, with a 20 ns decay time, can count 11.5 times faster thanNaI with a decay time of 230 ns. If everything else was equal and therewas no source output limit, this would mean that a tool with LaCl couldlog at 11.5 times the logging speed of a tool with NaI. In practice,everything else is not equal in these evaluations, hence thequantitative evaluation is preferably done looking at other factors.

TABLE 12 Summary of Detector Characteristics for some new ScintillatorsDensity % Relative Decay Time Cs FWHM Material (g/cc) P_(e) Light Output(ns) (%) NaI 3.67 336 100 230  6.5 BGO 7.13 1302  5.7 300 12.7 GSO(0.5%) 6.71 430 22 60  8.1 GSO (1.5%) 6.71 430 20 38  8.1 GSO-Z ~6.7  430 22 30  8.0 LuAP 8.30 763 12 17 14.9 LaCl₃ 3.64 281 64 20  4.8 LaBr₃5.30 244 122 35  3.6 La(Br,Cl)₃ ~4.5   ~260  ~150 ~25 ~3.8 LuAG:Pr 6.744 20 12  

Based on these arguments, the instantaneous countrates for a newgeneration tool are advantageously at least 4 to 6 times higher thanthat of existing tools. Therefore, any of the last 5 entries in Table 1would be good candidate detectors for this measurement.

It should be noted that for the PeX, the near spaced, back-scatterdetector in the density sub had too high a countrate for themeasurement. Because of this, GSO (1.5%)) in the table was developedthat had a faster decay time than regular GSO.

These fast detectors can also be applied to improvements in acceleratormeasurements producing neutrons in the following applications byreducing countrate losses, minimizing spectral distortion, and allowingmore detected counts, assuming neutron output can be increased: (1)Sigma Measurement; (2) Inelastic Spectroscopy Measurements; and (3)Capture Spectroscopy Measurements.

FIG. 3 is a plot illustrating how the decay time of LaCl can be modifiedby adjusting the concentration of Ce in the matrix. As the plot shows,curve 324 with the lowest Ce concentration has the slowest decay asindicated by the longer time required for the light signal to diminish.As the concentration of Ce is increased, the decay rate increases oncurves 322 and 320. The faster decay means that the detector is “reset”or ready to receive a new pulse earlier than with the lowerconcentration. As a result, this allows the detector to count faster, orprocess more counts per second.

Whereas many alterations and modifications of the present disclosurewill no doubt become apparent to a person of ordinary skill in the artafter having read the foregoing description, it is to be understood thatthe particular embodiments shown and described by way of illustrationare in no way intended to be considered limiting. Further, thedisclosure has been described with reference to particular preferredembodiments, but variations within the spirit and scope of thedisclosure will occur to those skilled in the art. It is noted that theforegoing examples have been provided merely for the purpose ofexplanation and are in no way to be construed as limiting of the presentdisclosure. While the present disclosure has been described withreference to exemplary embodiments, it is understood that the words,which have been used herein, are words of description and illustration,rather than words of limitation. Changes may be made, within the purviewof the appended claims, as presently stated and as amended, withoutdeparting from the scope and spirit of the present disclosure in itsaspects. Although the present disclosure has been described herein withreference to particular means, materials and embodiments, the presentdisclosure is not intended to be limited to the particulars disclosedherein; rather, the present disclosure extends to all functionallyequivalent structures, methods and uses, such as are within the scope ofthe appended claims.

What is claimed is:
 1. A system for detecting gamma-rays downholecomprising: a tool housing adapted and dimensioned to be deployed in aborehole within a subterranean formation; and a scintillator materialmounted within the tool housing, and emitting light when gamma-rays areabsorbed, the scintillator material having an associated decay time ofless than about 100 ns and an associated resonance integral of less thanabout 100 barns.
 2. A system according to claim 1 further comprising aphotodetector mounted within the tool housing and adapted so as todetect light emitted by the scintillator material.
 3. A system accordingto claim 1 wherein the system is adapted to detect gamma-rays primarilyproduced by inelastic scattering.
 4. A system according to claim 3wherein the system is adapted to reduce detection of gamma-rays producedby neutron capture events.
 5. A system according to claim 1 wherein thesystem is adapted to detect gamma-rays primarily produced by neutroncapture events.
 6. A system according to claim 1 further comprising anuclear source mounted within the tool housing and adapted so as to emitnuclear radiation into the subterranean formation.
 7. A system accordingto claim 6 wherein the nuclear source includes a neutron source adaptedto emit neutrons into the subterranean formation so as to producegamma-rays.
 8. A system according to claim 7 wherein the neutron sourceis an electronic neutron generator.
 9. A system according to claim 8wherein the system is adapted to detect gamma-rays primarily produced byinelastic scattering and reduce the sensitivity to epithermal neutronsto enhance the measurement of density.
 10. A system according to claim 6wherein the nuclear source includes an x-ray or gamma-ray source adaptedto emit x-rays or gamma-rays into the subterranean formation.
 11. Asystem according to claim 7 wherein the neutron source is a pulsedneutron source.
 12. A system according to claim 7 wherein the neutronsource includes a radioisotope source capable of emitting fast neutrons.13. A system according to claim 1 wherein the spacing between thenuclear source and the scintillator material is based at least in parton the selection of the scintillator material.
 14. A system according toclaim 1 further comprising a data processing system adapted andprogrammed to ascertain one or more properties of the subterraneanformation based at least in part on measurements of detected gamma-rays.15. A system according to claim 14 wherein the data processing system isfurther adapted and programmed to use an elemental yields type ofprocessing in ascertaining the one or more properties.
 16. A systemaccording to claim 1 wherein the scintillator material is primarily alanthanum halide material.
 17. A system according to claim 16 whereinthe scintillator material is primarily of a type selected from the groupconsisting of: LaCl₃, LaBr₃ and La(Br,Cl)₃.
 18. A system according toclaim 1 wherein the tool housing is adapted to be deployed in theborehole via a wireline cable.
 19. A system according to claim 1 whereinthe tool housing is adapted to be deployed in the borehole as part of adrill collar.
 20. A method for detecting gamma-rays downhole comprising:deploying a tool in a borehole within a subterranean formation; andmaking gamma-ray measurements using a scintillator material mounted inthe tool, the scintillator material emitting light when gamma-rays areabsorbed, and having an associated decay time of less than about 100 nsand an associated resonance integral of less than about 100 barns.
 21. Asystem for detecting gamma-rays downhole comprising: a tool housingadapted and dimensioned to be deployed in a borehole within asubterranean formation; and a scintillator material mounted within thetool housing and emitting light when gamma-rays are absorbed, and thescintillator material having an associated decay time of less than about40 ns.
 22. The system according to claim 21, wherein the scintillatormaterial is selected from a group consisting of: GSO-Z, LuAP, LaCl₃,LaBr₃, La(Br,Cl)₃, YAP, and LuAG.
 23. The system according to claim 21further comprising a photodetector mounted within the tool housing andadapted so as to detect light emitted by the scintillator material. 24.The system according to claim 21 wherein the system is adapted to detectgamma-rays primarily produced by neutron inelastic scattering.
 25. Thesystem according to claim 21 wherein the system is adapted to detectgamma-rays primarily produced by neutron capture events.
 26. The systemaccording to claim 21 wherein the system is adapted detect x-rays orgamma-rays scattered by the subterranean formation.
 27. The systemaccording to claim 21 wherein the system is adapted to detect gamma-raysprimarily produced by inelastic scattering and minimize the sensitivityto epithermal neutrons.
 28. The system according to claim 21 furthercomprising a nuclear source mounted within the tool housing and adaptedso as to emit nuclear radiation into the subterranean formation.
 29. Thesystem according to claim 28 wherein the nuclear source includes aneutron source adapted to emit neutrons into the subterranean formationso as to produce gamma-rays.
 30. The system according to claim 28wherein the nuclear source is an x-ray source adapted to emit x-raysinto the subterranean formation.
 31. The system according to claim 30wherein the x-ray source is an accelerator-based x-ray source.
 32. Adownhole tool comprising: a tool housing adapted and dimensioned to bedeployed in a borehole within a subterranean formation; an x-ray orgamma ray source mounted within the tool housing and adapted so as toemit x-rays or gamma rays into the subterranean formation; and ascintillator material mounted within the tool housing, and emittinglight when x-rays or gamma rays are absorbed, the scintillator materialbeing selected from a group consisting of: GSO-Z, LuAP, LaCl₃, LaBr₃,La(Br,Cl)₃ and LuAG.
 33. The downhole tool according to claim 32,wherein the scintillator material has an associated decay time that isshort enough to allow for processing of x-ray or gamma ray counts havingreduced losses and distortion.
 34. The downhole tool according to claim33, wherein the associated decay time is less than about 100 ns.
 35. Thedownhole tool according to claim 33, wherein the associated decay timeis less than about 40 ns.
 36. The downhole tool according to claim 32,wherein the x-ray source is an accelerator-based x-ray source.
 37. Thedownhole tool according to claim 32, wherein the gamma ray source is aradioactive gamma ray source.
 38. A method comprising: deploying a toolin a borehole within a subterranean formation; and detecting gamma-raysusing a scintillator material and a photodetector mounted within thetool, and the scintillator material having an associated decay time ofless than about 40 ns, the scintillator material emitting light whengamma-rays are absorbed, and the photodetector detecting light emittedby the scintillator material, and determining one or more properties ofthe subterranean formation based at least in part on the detectedgamma-rays.
 39. The method according to claim 38 wherein thescintillator material is primarily of a type selected from the groupconsisting of: LaCl₃, LaBr₃, La(Br,Cl)₃, YAP, and LuAG.
 40. The methodaccording to claim 38 wherein the determining comprising an elementalyields type of processing.